1. Field of the Invention
The present invention relates to a switching power supply, and in particular, to a quasi-resonance type switching converter that achieves reduction of surge voltage undergone by a rectifying diode and simultaneously ensures transient response.
2. Description of the Related Art
Quasi-resonance type switching converters are widely used for low power switching power supplies. This type of switching power supply, for example disclosed in Patent Document 1 (identified below), has a construction shown in FIG. 4. The switching power supply 1 of FIG. 4 comprises a switching element Q, which can be a MOS-FET, connected in series to a primary winding P1 of an isolation transformer T that receives an input voltage Vi, and a resonance capacitor Cr provided in parallel to the switching element Q. The switching power supply 1 also comprises an output capacitor Co connected through a rectifying diode D to a secondary winding S of the transformer T, and a control circuit 2 that is a control IC, for ON/OFF driving the switching element Q.
The control circuit 2 controls the process in which the switching element Q is turned ON to store electric energy from the input voltage Vi into the isolation transformer T, then the switching element Q is turned OFF to discharge the electric energy stored in the isolation transformer T from the secondary winding S of the isolation transformer T. Thus, output voltage Vo is obtained by rectification and smoothing with the rectifying diode D and the output capacitor Co.
The control circuit 2 receives a feedback voltage FB through a feedback circuit 3 including a photo-coupler; in which the FB voltage is a piece of information about the output voltage Vo detected by voltage dividing resistors Ro1 and Ro2. The control circuit 2 also receives an IS voltage that is detected through a shunt resistor Rs and corresponds to the current flowing through the switching element Q. The control circuit 2 further receives a ZCD voltage developed across an auxiliary winding P2 of the isolation transformer T accompanying ON/OFF operation of the switching element Q.
The control circuit 2 has a first comparator 12, a bottom voltage detecting circuit that compares the ZCD voltage given through a level-shift and delay circuit 11 with a reference voltage Vref1 and detects a bottom of the voltage developing at the isolation transformer T or, in other words, voltage undergone by the switching element Q, delivering a BOT signal. The control circuit 2 also has a second comparator 13, a current comparator, that compares the IS voltage with the FB voltage and, if the IS voltage has exceeded the FB voltage, delivers a reset signal. The control circuit 2 further has a third comparator 14 that delivers a reset signal when the FB voltage has decreased below a reference voltage Vref2. The control circuit 2 comprises an RS flip-flop 15 that is set by the BOT signal and reset by the reset signal, and delivers an output signal q. The output signal q is basically used as a control signal for ON driving the switching element Q through the driving circuit 16 when the RS flip-flop 15 is set and the output signal q is in an “H” state.
More specifically, the BOT signal is given through an inverter 17 to a clock terminal of a D flip-flop 18. This D flip-flop 18 is set by the BOT signal under the condition that the RS flip-flop 15 is reset, which means the switching element Q is in an OFF state. The set output of the D flip-flop 18 is given through an OR circuit 19 to the RS flip-flop 15, setting the RS flip-flop 15, to turn ON the switching element Q. Since the D flip-flop 18 sets the RS flip-flop 15 based on the BOT signal only when the RS-flip-flop 15 is reset, the D flip-flop 18 limits the maximum frequency of ON/OFF driving of the switching element Q.
With the ON operation of the switching element Q, the current flowing in the switching element Q through the isolation transformer T increases. When the IS voltage exceeds the FB voltage, the second comparator 13 delivers a reset signal. The reset signal is given to the RS flip-flop 15 through an OR circuit 20 resetting the RS flip-flop 15, to turn OFF the switching element Q. The third comparator 14 forcedly resets the RS flip-flop 15 through the OR circuit 20 when the FB voltage decreases in an event of abrupt load variation or burst operation.
The control circuit 2 is provided with a restart circuit 21 that sets the RS flip-flop 15 to resume ON/OFF drive of the switching element Q after continuation for a certain period of time Tres, 10 μs for example, in a state in which the output voltage Vo is abnormally decreased and the RS flip-flop 15 is reset, resulting in a halt of ON driving of the switching element Q.
The restart circuit 21 starts up when the switching element Q enters an OFF state in which the RS flip-flop 15 is in a reset state and the BOT signal is not delivered. After the period of time Tres, the restart circuit 21 delivers a restart signal to set the RS flip-flop 15. If the RS flip-flop 15 is set or the BOT signal is delivered during the operation period, the period of time Tres, of the restart circuit 21, the restart circuit 21 is initialized through an OR circuit 22. In other words, when the switching element Q is ON/OFF driven at an oscillation frequency in a specified range and consequently, the RS flip-flop 15 is set or the BOT signal is delivered, the restart circuit 21 is initialized. Therefore, the restart signal is never delivered as long as the switching element Q is ON/OFF driven at a specified oscillation frequency.
[Patent Document 1]                Japanese Unexamined Patent Application Publication No. 2011-015570.        
At start up time of the switching power supply having the construction described above, the ZCD voltage is zero and thus, the RS flip-flop 15 is set according to the restart signal delivered by the restart circuit 21 to ON drive the switching element Q. When the IS voltage exceeds the FB voltage, the RS flip-flop 15 is reset to turn OFF the switching element Q. Thus, the output capacitor Co is charged with the electric energy discharged from the secondary winding S of the isolation transformer T.
In the startup period of the switching power supply, however, the charged voltage that is the output voltage Vo is approximately zero, resulting in an elongated discharge time Tdis, for example 100 μs, of discharging the electric energy from the secondary winding S. At this time, the BOT signal remains at the “L” level due to a low value of the ZCD voltage itself; thus the restart circuit 21 starts to operate correspondingly to reset of the RS flip-flop 15 accompanying the turning OFF of the switching element Q. After the period of time Tres, the restart circuit 21 delivers a restart signal again, which in turn sets the RS flip-flop 15 to turn ON the switching element Q.
At this moment of turning ON of the switching element Q, however, discharge of electric energy is not completed from the secondary winding S of the isolation transformer T, that is, Tres<Tdis, and a reverse recovery current is generated in the rectifying diode D. FIG. 5 illustrates waveforms of the drain voltage Vds of the switching element Q, the current Idiode through the rectifying diode D, and the anode voltage Vdiode of the rectifying diode D during a period of ON/OFF operation of the switching element Q. FIG. 5 clearly shows a large surge voltage Vsurge developed across the rectifying diode D.
In order to suppress the generation of surge voltage Vsurge, a means can be employed to set the restart period of time Tres to be longer than the discharge time Tdis, in which the period of time Tres is the time duration from a start up of the restart circuit 21 accompanying turning OFF of the switching element Q to delivery of a restart signal, and the time Tdis is a period of time for discharging the electric energy from the secondary winding S. Setting the period of time Tres to be long remarkably deteriorates transient response when the FB voltage becomes lower than the reference voltage Vref2 due to abrupt change in the load during normal operation after the end of a start up period of the switching power supply.